`defer` doesn't define a new kind of observable, but allows you to declare that a source observable should be created when a subscriber arrives. Consider how you would create an observable that returns the current time and terminates. You are emitting a single value, so it sounds like a case for `just`.

Observable<Long> now = Observable.just(System.currentTimeMillis());

now.subscribe(System.out::println);

Thread.sleep(1000);

now.subscribe(System.out::println);

Output

Notice how the two subscribers, 1 second apart, see the same time. That is because the value for the time aquired once, when the observable is created. What you want is for the time to be aquired when a subscriber asks for it.

Observable<Long> now = Observable.defer(() ->

Observable.just(System.currentTimeMillis()));

now.subscribe(System.out::println);

Thread.sleep(1000);

now.subscribe(System.out::println);

Output